On Thu, Jul 19, 2018 at 09:20:34AM +0200, Markus Armbruster wrote: > Peter Xu <pet...@redhat.com> writes: > > > On Wed, Jul 18, 2018 at 05:38:11PM +0200, Markus Armbruster wrote: > >> Peter Xu <pet...@redhat.com> writes: > >> > >> > After the Out-Of-Band work, the monitor iothread may be accessing the > >> > cur_mon as well (via monitor_qmp_dispatch_one()). Let's convert the > >> > cur_mon variable to be a per-thread variable to make sure there won't be > >> > a race between threads when accessing the variable. > >> > >> Hmm... why hasn't the OOB work created such a race already? > >> > >> A monitor reads, parses, dispatches and executes commands, formats and > >> sends replies. > >> > >> Before OOB, all of that ran in the main thread. Any access of cur_mon > >> should therefore be from the main thread. No races. > >> > >> OOB moves read, parse, format and send to an I/O thread. Dispatch and > >> execute remain in the main thread. *Except* for commands executed OOB, > >> dispatch and execute move to the I/O thread, too. > >> > >> Why is this not racy? I guess it relies on careful non-use of cur_mon > >> in any part that may now execute in the I/O thread. Scary... > > > > I think it's because cur_mon is not really used in out-of-band command > > executions - now we only have a few out-of-band enabled commands, and > > IIUC none of them is using cur_mon (for example, in > > qmp_migrate_recover() we don't even call error_report, and the code > > path is quite straight forward to make sure of that). So IIUC cur_mon > > variable is still only touched by main thread for now hence we should > > be safe. However that condition might change in the future when we > > add more out-of-band capable commands. > > > > (not to mention that I don't even know whether there are real users of > > out-of-band if we haven't yet started to support that for libvirt...) > > It's not just the actual OOB commands (there are just two), it's also > the monitor code to read, parse, format and send.
My understanding is that read, parse, format, send will not touch cur_mon (it was touched before but some patches in the out-of-band series should have removed the last users when parsing). So IIUC only the dispatcher would touch that now. I didn't consider the callers like net_init_socket() and I'm only considering the monitor code (and those callers should be only in the main thread too after all). > > >> Should this go into 3.0 to reduce the risk of bugs? > > > > Yes I think it would be good to have that even for 3.0, since it still > > can be seen as a bug fix of existing code. > > Agreed. > > > Regards, > > > >> > Note that thread variables are not initialized to a valid value when new > >> > thread is created. > > Confusing. It sounds like @cur_mon's initial value would be > indeterminate, like an automatic variable's. Not true. Variables with > thread storage duration are initialized when the thread is created. > Since @cur_mon's declaration lacks an initializer, it'll be initialized > to a null pointer. Your sentence is correct when you consider that null > pointer not a valid value. Yes that's what I meant. So how about this? Note that the per-thread @cur_mon variable is not initialized to point to a valid Monitor struct when a new thread is created (the default value will be NULL). Please feel free to tune it up. > > >> > However for our case we don't need to set it up, > >> > since the cur_mon variable is only used in such a pattern: > >> > > >> > old_mon = cur_mon; > >> > cur_mon = xxx; > >> > (do something, read cur_mon if necessary in the stack) [1] > >> > cur_mon = old_mon; > >> > > >> > It plays a role as stack variable, so no need to be initialized at all. > >> > We only need to make sure the variable won't be changed unexpectedly by > >> > other threads. > > Do we need this paragraph? The commit doesn't mess with @cur_mon's > initial value at all... I was trying to explain why we don't need to initialize that variable for each thread. A common idea (at least that's what I have had in mind) is that when we create a new thread we should possibly inherit that @cur_mon variable in a copy-on-write fashion for that new thread. But that's not really necessary for the use case like above (as long as we don't create thread during [1], and that's what we do). If you think the patch explains itself better without these lines, please feel free to drop it. > > >> > Reviewed-by: Eric Blake <ebl...@redhat.com> > >> > Reviewed-by: Marc-André Lureau <marcandre.lur...@redhat.com> > >> > Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com> > >> > [peterx: touch up commit message a bit] > >> > Signed-off-by: Peter Xu <pet...@redhat.com> Thanks, -- Peter Xu
